PREHEAT oven to 325º F. Line 8-inch-square baking pan with foil; grease.
HEAT 10 ounces (2 1/2 bars) chocolate (broken into small pieces), sugar, butter and water in small, heavy-duty saucepan over low heat, stirring constantly, until chocolate and butter are melted. Pour into medium bowl. Stir in eggs, one at a time, until mixed in. Stir in vanilla extract. Add flour and salt; stir well. Stir in nuts, if desired. Pour into prepared baking pan.
BAKE for 35 to 40 minutes until wooden pick inserted in center comes out slightly sticky (may be up to 45 minutes). Cool in pan on wire rack. Lift brownie from pan with foil to cutting board. Carefully remove foil. Cut brownie square into half. Cut each half into thirds, for a total of 6 pieces. Cut each piece in half diagonally to form triangles for a total of 12.
PLACE cream in small, uncovered, microwave-safe dish. Microwave on HIGH (100%) power for 25 to 30 seconds. Add remaining 2 ounces (1/2 bar) chocolate, broken into small pieces; stir until smooth. (Sauce will thicken as it cools.) Place wedge on serving plate; top or drizzle with a teaspoon of sauce. Top with whipped cream, if desired.